Past coal-fired generation activities have resulted in CCR sediments in disposal ponds or impoundments. These CCR ponds require closure to mitigate their impact on the neighboring environment and human or animal health. Closure is also now required by U.S. environmental regulation. However, to facilitate closure, the CCR ponds are sometimes dewatered by pre-drainage of the CCR to enhance strength and stability of the material and thereby provide a stable surface on which to operate earthmoving and grading equipment. If pre-drainage (e.g., by pumping wellpoints installed in the CCR to lower the groundwater table) does not sufficiently improve strength and stability of the in-place CCR due to its drainage properties, it becomes necessary to improve CCR strength and stability with admixtures such as quicklime, dry fly ash, or Portland cement; evaporative drying in place, or by dredging or excavating the CCR, dewatering it to consolidate it and improve its strength and handling characteristics, and landfilling it either in the same place or by hauling it a different disposal location.
CCR is known to be unstable when saturated. When saturated CCR is subject to shear strain, it densifies and expels water, resulting in a near total loss of shear strength. In this state, the material becomes a viscous fluid and may begin to slide or flow. This process may result in overtopping of impoundments and makes excavation and handling difficult to impossible. Reducing the water content of the CCR material by only a few percentage points has a dramatic effect on its behavior, allowing stable, near vertical cuts suitable for mass excavation.
Dewatering methods include both mechanical dewatering and geotube dewatering. In mechanical dewatering, dredged CCR is pumped to a mechanical dewatering unit (e.g., a centrifuge, a belt press, or a filter press), dewatered, and the filtered CCR (filter cake) is placed in a landfill. Often, the filtered CCR cake requires solidification/stabilization because it cannot support earthwork equipment that is used on the surface of landfills.
Geotube dewatering uses geotubes for dewatering. Geotubes are large filter bags made of geotextile. Dredged CCR is pumped into a geotube and the water is allowed to drain, leaving CCR solids in the geotube. After the geotube is filled with dredged CCR, it is allowed to drain for some time. When the geotube collapses as water is drained, more dredged CCR is pumped into the geotube. After cycles of filling and draining, the geotube is filled with “drained” CCR. The drained CCR may be dewatered further, if desired, by evaporative drying for several weeks. The dewatered CCR may be taken off site for disposal or disposed of in an on-site landfill.
Consolidation refers to a process of subjecting the CCR to a load so that the CCR undergoes volume reduction and strength gain as a result of water being effectively forced out of the loaded CCR volume. Since CCR does not allow water to flow out easily due to its very low hydraulic conductivity, drainage pathways are provided in the CCR volume to accelerate consolidation. The most common way of providing drainage pathways is to insert prefabricated drains vertically into the CCR. The prefabricated drains consist of a plastic core wrapped with geotextile filter which, when installed in the CCR, facilitates the flow of water into the drain and to the surface of the ground. Prefabricated drains can consist of flat plastic cores with a geotextile envelope, commonly installed using a hollow rectangular mandrel that is pressed into the ground, or perforated circular plastic pipe/tube surrounded by a geotextile envelope, installed by drilling an open hole with drilling fluid, or jetting or driving an open-ended temporary steel casing/tube or advancing a continuous hollow auger and inserting the perforated plastic pipe or tube and geotextile envelope before the temporary casing/tube or hollow auger is extracted.